Solid propellant compositions having a nitrocellulose-hydroxyl-terminated polybutadiene binder and method of preparing the same

ABSTRACT

IMPROVED NITROCELLULOSE-BASE COMPOSITIONS OBTAINED BY INCORPORATING HYDROXYL-TERMINATED POLYBUTADIENE POLYMER IN THE NITROCELLULOSE BINDER. PROPELLANT COMPOSITIONS BASED ON THE RESULTING BINDER EXHIBIT SUBSTANTIALLY IMPROVED STRENGTH AND THERMAL STABILITY.

SOLID PROPELLANT COMPOSITIONS HAVING A NITROCELLULOSEHYDROXYL-TERMINATED POLYBUTADIENE BWDER AND METHOD OF PREPARING THE SAMEEverette M. Pierce, Somerville, Ala., assignor to the United States ofAmerica as represented by the Secretary f the Army No Drawing. FiledJune 30, 1965, Ser. No. 469,975

Int. Cl. (106d /.06

U.S. Cl. 149-49 8 Claims ABSTRACT on THE DISCLOSURE Improvednitrocellulose-base compositions obtained by incorporatinghydroxyl-terminated polybutadiene polymer in the nitrocellulose binder.Propellant compositions based on the resulting binder exhibitsubstantially improved strength and thermal stability.

The invention described herein may be used by or for the Government forgovernmental purposes without the payment of any royalty thereon.

This invention relates to improved nitrocellulose-base compositions andto a method of preparing the same.

Numerous rocket motors now in use or under development usenitrocellulose-base compositions as solid propellants. As used herein,the term nitrocellulose-base composition refers to any compositioncontaining plasticized nitrocellulose. In such compositionsnitrocellulose, normally present at a proportion of 5 to 40 weightpercent, serves as a polymeric binder or base material, and aplasticizer is used to produce the desired plastic solid structure.Nitrocellulose-base compositions containing an explosive nitrate esterplasticizer such as nitroglycerin are designated double-basecompositions. When high energy fuel and oxidizer materials such aspowdered aluminum and ammonium perchlorate are provided in a double-basecomposition, the composition is designated composite double-base.

One of the major problems presented in the use of nitrocellulose-basecompositions is attainment of suitable physical properties over a widetemperature range. A satisfactory solid propellant should be flexibleenough to withstand handling and firing at low temperatures such as 4()F. without fracturing the grain. The propellant should also havesufiicient thermal stability to avoid melting, deformation ordecomposition at temperatures up to at least 140 F., and preferably upto 200 F. Stability of properties over this range of temperatures isrequired to permit handling and storage of the propellant under avariety of field conditions.

Previously known nitrocellulose-base compositions have not proven fullysatisfactory in melting these temperature requirements. Many of thesecompositions have been too brittle to permit handling at the lowertemperatures with out fracture. Improved temperature-dependentproperties have been attained by various methods such as using higherproportions of plasticizer and a ferrocene curing agent, but furtherimprovements are desired. In the case of double-base compositecompositions, low-temperature strength and high-temperature stabilityare adversely afiected by the presence of high proportions of fuel andoxidizer additives, the proportions of nitrocellulose binder andplasticizer which form the matrix of the composition beingcorrespondingly decreased. It is desired to provide a means of improvingthe strength and stability of doublebase composite compositions byimproving the nitrocellulose binder component. This approach would allowthe use of higher proportions of fuel and oxidizer additives in a givencomposition.

3,725,729 Patented Apr. 10, 1973 It is therefore an object of thisinvention to provide improved nitrocellulose-base compositions.

Another object is to provide a method for improving the low-temperaturemechanical properties of nitrocellulose-base compositions.

Still another object is to provide a method for increasing the thermalstability of nitrocellulose-base compositions.

Yet another object is to provide a thermally stable double basecomposite composition containing relatively high proportions of fuel andoxidizer additives.

Other objects and advantages of this invention will be apparent from thefollowing detailed description.

In the present invention improved nitrocellulose-base compositions areobtained by incorporating hydroxylterminated polybutadiene polymer inthe nitrocellulose binder. The hydroxyl-terrninated polybutadieneimparts greater strength and thermal stability to the resulting bindercomponent, probably by cross-linking of the nitrocellulose andhydroxyl-terminated polybutadiene polymer chains. Compositions based onthis binder, and particularly double-base composite propellants, exhibitsubstantially improved properties, both at low and high temperatures inthe desired operating range.

Although this invention is not to be understood as limited to aparticular theory, it is postulated that the hydroxyl groups of thehydroxyl-terminated polybutadiene are cross-linked with nitrocelluloseby attachment at the sites on the nitrocellulose molecule havingresidual hydroxyl groups. Nitrocellulose is normally not fully nitratedto the theoretical composition cellulose trinitrate, and such residualhydroxyl groups are available for crosslinking. In eifect, thehydroxyl-terminated polybutadiene reinforces and strengthens thenitrocellulose molecular structure.

Hydroxyl-terminated polybutadiene for use in the present invention maybe represented by the following formula:

Where n is from 45 to 80. The molecular weight of the polymers includedwithin this formula is from about 2500 to 4000. Commercially availablehydroxyl-terminated polybutadiene polymer may be used, or this materialcan be prepared by reduction of the corresponding carboxylterminatedpolybutadiene. Carboxyl-terminated polymer is not suitable for thepresent invention in that it causes excessive gassing during curin'resulting in an unsatisfactory porous product. The amount ofhydroxyl-terminated polybutadiene may be varied over a wide range so asto provide a hydroxyl-terminated polybutadiene-tonitrocellulose weightratio from about 1:8 to 3:1. The amount employed for a particularcomposition is determined by the properties required in the composition.Higher proportions of hydroxyl-terminated polybutadiene result in bettertemperature-dependent physical properties, but the total energy of thecomposition is decreased. For high-energy composite double-basepropellants, a ratio as defined above of about 1:4 is preferred toobtain substantially improved properties, consistent with a high energycontent.

Nitrocellulose having a nitrogen content from 11 to 13 weight percentmay be used in this invention, and about 12.6 weight percent nitrogencontent is preferred. The nitrogen content of the nitrocellulosenormally used for propellants falls within this range so that nodifiiculty is presented by this requirement. Nitrocellulose nitrated tothe maximum theoretical extent would not be eifective since no hydroxylsites would be available for cross-linkmg.

Formation of the nitrocellulose-hydroxyl-terminated polybutadiene binderrequires use of a cross-linking agent. Any of the diisocyanatespreviously used for cross-linking nitrocellulose or hydroxyl-terminatedpolybutadiene can be used for this purpose. Examples of suitablecross-linking agents are toluene 2,4-diisocyanate and hexamethylenediisocyanate, the latter being preferred. The cross-linking agent ispreferably supplied at a proportion by weight of about 1:8 to 1:15 withrespect to the total content of nitrocellulose and hydroxyl-terminatedpolybutadiene.

In its broadest scope, the present invention is applicable to anynitrocellulose-base composition, the novelnitrocellulose-hydroxyl-terminated polybutadiene polymer being used insubstantially the same manner as nitrocellulose alone in previouscompositions. In general, nitrocellulose-base compositions are preparedby mixing nitrocellulose with a plasticizer to form a gelatinizedcolloidal mixture and curing the mixture by application of mild heat toproduce a tough, rubbery solid. Any desired additives such as additionalfuel or oxidizer materials, catalysts and stabilizers are incorporatedin the composition prior to curing. The nitrocellulose-polybutadienebinder is compatible with all of the additives employed in previouslyknown nitrocellulose-base compositions. All that is required in thepresent invention is curing of the nitrocellulose-hydroxyl-terminatedpolybutadiene containing composition in the presence of a cross-linkingagent.

Any of the Well-known plasticizers for nitrocellulose can be used forcompositions containing the nitrocellulosehydroxyl-terminatedpolybutadiene binder. Illustrations of these plasticizers are nitrateesters (e.g., nitroglycerin, hydroxyglycerol trinitrate, butane trioldinitrate, diethylene glycol dinitrate, triethylene glycol dinitrate,trimethylene trinitrate and mixtures of two thereof) dibutyl phthalate,dimethyl sebacate, dibutyl succinate, dibutyl adipate, triacetin, ethyldiphenyl phosphate, tributyl phosphate, tetrazole derivatives as setforth in 1.1.8. Pat. 3,073,731 and dinitriles as set forth in US. Pat.3,104,190 and mixtures thereof. The nitrate esters are the preferredclass of plasticizers. Although not critical to the present invention,the amount of plasticizer employed in nitrocellulose-base compositionsis normally about to 50 weight percent.

When an explosive plasticizer, such as nitroglycerin, is used innitrocellulose-base compositions contemplated by the present invention,the nitrocellulose and plasticizer contain sufiicient oxidizer to assuresubstantially complete combustion. To obtain higher energy, however,additional fuel and oxidizer materials can be incorporated in thecomposition. Organic oxidizers such as pentaerythritol tetranitrate,cyclotetramethylenetetramine, cyclotrimethylenetrinitramine andtetranitrobutane's or inorganic oxidizers such as ammonium perchlorate,ammonium nitrate and alkali and alkaline earth metal chlorates,perchlorates and nitrates can be employed. Up to about 70 weightpercent, oxidizer can be used, but less than 60 weight percent ispreferred in order to maintain good mechanical properties.

Powdered metal additives can be provided in nitrocellulose to serve ashigh-energy fuel component; aluminum, zirconium, lithium, magnesium,boron and their hydride's as Well as alloys of these metals can be usedfor this purpose. Metal additives normally constitute less than 30weight percent of the cured composition.

Other additives which may be used in nitrocellulosebase compositionswithin the scope of this invention include, but are not limited to,burning rate modifiers exemplified by iron powder, lead salicylate, leadoxide, lead chromate, lead stearate, lead oxide and ammoniumperchlorate; stabilizers exemplified by symmetrical diethyl diphenylurea, resorcinol and 2-nitrodiphenylamine; pigments or fillersexemplified by zinc oxide, carbon blacks, titanium dioxide, candellilawax and sericite. The amount of such additives may be varied, dependingon the specific Permissible Optimum Component proportion proportionNitrocellulose 540 8-10 Diisocyanate 0. 55 1-1. 5 IIydroxyl-terminatedpolybutadiene 1-15 24 Plasticizer 20-50 30 35 5-70 30-40 1-30 15-21Ammonium perchlorate and aluminum are the preferred oxidizer and fuel,respectively, in this embodiment, and nitrate esters are the preferredgroup of plasticizers. In addition, these compositions normally contain1 to 5 weight percent of additivies such as the burning rate modifiers,stabilizers, pigments or fillers described above. The low-temperaturestrength and high-temperature stability of these compositions aresubstantially improved over prior compositions of this type havingnitrocellulose alone as the binder.

The compositions described above are prepared by thoroughly mixing thespecified ingredients and curing the resulting mixture to obtain atough, rubbery solid material. The order of adding the variousingredients is not critical; but, in the case of composite double-basepropellants, it is preferred to blend all dry materials except theoxidizer before mixing with the liquid components. Formation of thenitrocellulose-hyclroxyl-terminated polybutadiene copolymer occursduring curing. The mixture is cured by subjecting it to mild heat for anextended period, a suitable curing cycle comprising holding the mixtureat a temperature of about 135 F. for five days.

Propellant grains of the desired shape and size can be formed by pouringthe mixture into a mold or rocket motor case prior to curing, themixture preferably being heated to a temperature of to F. for betterfluidity in pouring. It is to be understood that this procedure is notcritical, and that fabrication methods used for previously known solidpropellants can in general be used for the present compositions. Theonly substantial difference is that the hydroxyl-terminatedpolybutadiene-containing compositions require a longer curing period,for example, five days as compared with two days for similarnitrocellulose-base compositions without this component.

This invention is further illustrated by the following example.

EXAMPLE I Composite double-base propellants with and withouthydroxyl-terminated polybutadiene were prepared to determine the effectof this component on temperaturedependent properties. In each case a onekilogram sample was prepared by mixing the specified ingredients,pouring the mixture into a mold and curing for 5 days at F.

High-temperature thermal stability of each sample was determined bymeans of a standard 120 C. deflagration test wherein the sample is heldat 120 C. until deflagration, or self-ignition at a constanttemperature, occurs.

Maximum tensile stress in pounds per square inch and strain in percentelongation were determined at 40 F. and F. by conventional methods.Specific impulse was determined for two samples by firing in a teststand.

The composition in weight percent of each sample and the resultsobtained may be seen by reference to the following table.

TESTING OF PROPELLANI COMPOSITIONS Sample Number 1 2 3 4 Component:

Ball powder (90% nitrocellulose,

8% nitroglycerin, 2% 2-nitrodiphenylaminc) 16. 7 Resorcinol Aluminum.

0 Tiiethylene glycol dinitrate 37. 3 36.

Strain at maximum stress,

140 F., percent elongation. 1 Maximum tensile stress, 40

F., pounds per square inch 579 Maximum tensile stress, 140F.,

poundspersquareinch 45 Specific impulse, pounds thrust per pound ofpropellant It may be readily seen that thermal stability was greatlyimproved for the samples containing hydroxyl-terminated polybutadiene,the time to deflagration being increased from 550 and 600 seconds toover 2000.

With respect to physical properties at low and high temperatures,substantial improvement was obtained for the most critical properties,namely, strain at -40 F. and tensile stress at 140 F. The lowtemperature strain value indicates the extent to which the propellantgrain would be subject to brittle fracture. This value was increasedfrom 13 and 7 percent to 22 and 16 percent, respectively, by theaddition of hydroxyl-terminated polybutadiene. The stress value at 140F. indicates the strength and rigidity of the propellant, a relativelyhigh value being required to avoid flowing at high tempera tures.Increases of from 45 to 77 and from 27 to 85 were obtained for thisvalue by addition of hydroxyl-terminated polybutadiene. The values forstrain at 140 F. and stress at 40 F. were not improved in all cases, butthese values are not nearly as critical to propellant performance, andthe values obtained for samples 2 and 4 are satisfactory. The specificimpulse of propellant compositions is only slightly decreased byaddition of hydroxylterminated polybutadiene, as evidenced by thedecrease from 248 for sample 3 to 246 for sample 4.

The above examples are merely illustrative and are not to be understoodas limiting the scope of this invention, which is limited only asindicated by the appended claims.

What is claimed is:

1. A binder for plasticized nitrocellulose-base compositions comprisinga copolymer of nitrocellulose and hydroxyl-terminated polybutadiene.

2. A propellant composition comprising a cured intimate mixture ofnitrocellulose, hydroxyl-terminated polybutadiene, a plasticizer and across-linking agent, the weight ratio of hydroxyl-terminatedpolybutadiene to nitrocellulose being from 1:8 to 3:1.

3. A propellant composition comprising a cured intimate mixture ofnitrocellulose, hydroxyl-terminated polybutadiene, a nitrate esterplasticizer and a diisocyanate cross-linking agent, the weight ratio ofhydroxyl-terminated polybutadiene to nitrocellulose being from 1:8 to3:1.

4. A composite double-base propellant composition comprising a curedintimate mixture of 5 to 40 weight percent nitrocellulose, 1 to 15weight percent hydroxylterminated polybutadiene, 0.5 to 5 weight percentdiisocyanate, 20 to 50 weight percent plasticizer, 5 to weight percentoxidizer and 1 to 30 weight percent fuel.

5. A composite double ba'se propellant comprising a cured intimatemixture of 5 to 40 weight percent nitrocellulose, 1 to 15 weight percenthydroxyl-terminated polybutadiene, 0.5 to 5 weight percent diisocyanate,20 to 50 weight percent nitrate ester plasticizer, 5 to 70 weightpercent inorganic oxidizer and 1 to 30 weight percent of a fuelcomponent selected from the group consisting of aluminum, zirconium,lithium, boron, magnesium and alloys and hydrides thereof.

6. A propellant composition comprising a cured intimate mixture of 5 to40 weight percent nitrocellulose, 0.5 to 5 weight percent diisocyanatecross-linking agent, 1 to 15 weight percent hydroxyl-terminatedpolybutadiene, 20 to 50 weight percent nitrate ester plasticizer, 5 to70 weight percent ammonium perchlorate and 1 to 30 weight percentpowdered aluminum.

7. A propellant composition comprising a cured intimate mixture of 8 to10 weight percent nitrocellulose, 1 to 1.5 weight percent diisocyanatecross-linking agent, 2 to 4 weight percent hydroxyl-terminatedpolybutadiene, 30 to 35 weight percent nitrate ester plasticizer, 30 to40 weight percent ammonium perchlorate, and 15 to 21 weight percentpowdered aluminum.

8. The method of preparing a nitrocellulose-base composition whichcomprises bringing together in intimate admixture nitrocellulose,hydroxyl-terminated polybutadiene, a diisocyanate cross-linking agentand a plasticizer and curing the resulting mixture, the proportion byweight of hydroxyl-terminated polybutadiene to nitrocellulose in theresulting mixture being from 1:8 to 3: 1.

References Cited UNITED STATES PATENTS BENJAMIN R. PADGETT, PrimaryExaminer US. Cl. X.R.

